Microwave magnetron

ABSTRACT

A combined RF seal, magnetic path and airflow path-defining means are provided for permanent-magnet-type magnetrons utilized in microwave heating. Spacer members of a ferromagnetic material are disposed between magnetic field pole piece means and the magnet means to assist in the magnetic field orientation and provide for improved circulation of a fluid medium. The efficiency of the magnetic circuit is enhanced by the more efficient cooling of magnet members. Any RF energy leakage along the electrical leads connecting the magnetron to the high voltage electric supply is also effectively suppressed.

llnited States Patent 91 Mim s June 12, 1973 [54] MICROWAVE MAGNETRON 3,588,589 6/1971 Vonk 315/3953 [75] Inventor: James R. Mims, Acton, Mass.

v Primary Examiner-Rudolph V. Rollnec Asslgnee: Raytheml p Lexmgton, Assistant Examiner-Saxfield Chatmon, Jr.

M Attorney-Harold A. Murphy et al. [22] Filed: Apr. 24, 1972 211 Appl. No.: 247,080 [57] ABSTRACT A combined RF seal, magnetic path and airflow pathdefining means are provided for permanent-magnet- F 'i 315/3951 i i g -g type magnetrons utilized in microwave heating. Spacer g l gi 51 members ofa ferromagnetic material are disposed be- 1 0 can tween magnetic field pole piece means and the magnet means to assist in the magnetic field orientation and provide for improved circulation of a fluid medium. [56] References cued The efficiency of the magnetic circuit is enhanced by UNITED STATES PATENTS the more efficient cooling of magnet members. Any RF 3,562,579 '2/1971' Kakizawa 315/39.71 energy leakage along the electrical leads connecting 3,304,400 2/1967 ojelid 315/3971 X the magnetron to the high voltage electric supply is also 3,577,033 5 1971 Aoki et al. sis/39.71 ff ti l suppress 3,493,810 2/1970 Valles 3l5/39.71 X 3,588,588 6/1971 Numata BIS/39.53 6 Claims, 5 Drawing Figures 82 se 76 s Patented June 12, 1973 3,739,225

3 Sheets-Sheet 1 Patent ed June 12, 1973 3 Sheets-Sheet 2 HIGH VOLTAGE SUPPLY \\\\\\\\\\\\\\\T\1J E Patented June 12, 1973 3 Sheets-Sheet {5 1 MICROWAVE MAGNETRON BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to microwave magnetrons and, more particularly, to the permanent-magnet-type.

2. Description of the Prior Art The magnetron has received wide usage in the generation of high frequency electromagnetic energy in microwave ovens. The device utilizes crossed electric and magnetic fields extending within an interaction region defined between a central cathode assembly with a heater and circumferentially disposed anode cavity resonators. Magnetic field producing means such as an electromagent are utilized in such magnetrons. The electrical operating voltages applied between the anode and the cathode are generated by means of DC rectification of the AC line voltages and run as high as 6,000 volts. The energy is coupled from the magnetron by means of an output member positioned within a waveguide launching section communicating with the oven enclosure. The text Microwave Magnetrons, Radiation Laboratory Series, Vol. 6, by GE. Collins, McGraw-Hill Book Co., Inc., 1948, may be'referred to for further details relative tothe magnetron energy generators. In addition, an example of a prior art microwave oven magnetron is disclosed in U.S. Pat. No. 3,531,613 issued Sept. 29, 1970, to CI. Domenichini et al and assigned to the assignee of the present invention.

The high electrical voltages present in the circuit for operation of the magnetron present an inherent safety as well as interference problem with adjacent communications equipment such as television, UHF and shortwave apparatus. Certain controls and standards, therefore, have been established .by state and federal regulatory agencies and bodies, such as the Federal Communications Commission and the Department of Health, Education and Welfare. These standards are in addition to the standards for the regulation of the escape of the RF energy generated within the oven enclosure during operation. Certain frequency bands are allocated in the low and high frequency electromagnetic energy spectrum, for example, 915 MHz which is widely utilized for industrial microwave heating and the more commonly employed band of 2450 MHz for the domestic ovens. Numerous police and government communications operate close to these frequencies and would be subject to any interference by energy leakage over the external leads coupling the magnetron to the high voltage circuits. Harmonic frequencies of the main operating frequency also interfere with communications systems-and equipment. The term microwave as used in this description of the invention is intended to refer to that portion of the electromagnetic energy spectrum having wavelengths in the order of 30 centimeters to one milimeter and frequencies in excess of 300 MHz.

In recent years microwave magnetrons have evolved utilizing integral permanent-magnet means which obviates the requirement for a separate voltage source for the operation of the solenoidelectromagnet structures. Inexpensive magnets are readily available in the commercial market such as the type utilized for audio speakers. The reduction in weight as well as cost has resulted in an increased population of such magnetron devices in microwave heating. An example of a permanent-magnet-type magnetron is shown in copending application Ser. No. 152,309 filed June 11, 1971, by R. A. Foerstner, et a1. and also assigned to the assignee of the present invention. A problem in the integral permanent magnet structure arises, however, due to the generation of heat in the vicinity of the permanent magnets which reduces the magnetic flux characteristics. This leads to the desirability of improved means for increasing the efficiency of the permanent magnet magnetic circuit producing means. In addition, the placement of the permanent magnets in the vicinity of the cathode and heater supporting structures with the accompanying external electrical leads requires the provision of a passageway within the magnet bodies which can result in RF leakage to cause interference in neighboring electronic equipment.

A need exists, therefore, for improved cooling means for permanent-magnet-type magnetrons as well as means for suppression of RF leakage over the cathodeheater electrical leads. Additionally, means for enhancing the flux density will increase the efficiency of microwave magnetrons.

SUMMARY OF THE INVENTION In accordance with the invention spacer members of a ferromagnetic material are positioned adjacent to the end of the magnetron generator through which the external electrical leads extend. Such members may be disc-shaped or may be suitably shaped from an integral one-piece member. The spacer members abut a pole piece member defining a part of the magnetic return path to which an RF filter box enclosureand the permanent magnets are appended. The spacing of these members defines a plurality of paths perpendicular to the magnetron axis through which air may be directed in contact with the permanent magnets as well as the area surrounding the cathode-heater supporting structure for more efficient cooling. The spacer members abutting the magnetic pole piece plate members also provide a means for directing the magnetic field flux lines in the coaxial direction to extend parallel to the magnetron axis. The components envisaged in the practice of the invention are of a relatively low cost and require a minimum of tooling expense.

BRIEF DESCRIPTION OF THE DRAWINGS Details of the invention will be readily understood after consideration of the following description and reference to the accompanying drawings, wherein:

FIG. 1 is an isometric view, partly secioned, of an illustrative permanent-magnet-type magnetron embodying the invention;

FIG. 2 is an isometric view, with a wall partly broken away, of a microwave oven embodying the magnetron of the invention;

FIG. 3 is a vertical cross-sectional view of the embodiment shown in FIG. 2;

FIG. 4 is an exploded isometric view of the principal components of the illustrative embodiment of the invention; and

FIG. 5 is a plan view of an alternative embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, FIG. 1 illustrates a permanent-magnet-type magnetron l embodying the invention. Before proceeding to the detailed description, reference is directed to an exemplary microwave oven shown in FIGS. 2 and 3.

The microwave oven 12 comprises conductive walls 14 defining the enclosure 16 which has an access opening adapted to be closed by means of door assembly 18 which is manually actuated by handle 20. The door may be sliding, side-hinged or bottom-hinged. Control panel 22 is disposed adjacent to the door assembly. The

is coupled by means of antenna probe 36 disposed within a dielectric dome 38 which extends through an aperture in bottom wall 42 of a rectangular waveguide launching section 40. The section is closed at one end and the energy is coupled into the enclosure 16 through the inner open end 44. Multi-mode distribution of the energy is accomplished by any of the well-known means in the art including, for example, a mode stirrer 46 having a plurality of paddles 48 actuated by a fractional-horsepower motor 50.

Referring again to FIG. 1 the illustrative permanenttype-magnet magnetron will be described. Conductive envelope 52 houses the anode having a plurality of circumferentially disposed cavity resonators, as well as the central cathode and heater assembly. The cathode and heater leads 54 and 56 are disposed within and supported by a dielectric support assembly 58 which is axially disposed at the end of the envelope 52 opposite to the output coupling assembly 36, 38. The electrical leads 54 and 56 are coupled to the high voltage circuit through an RF shielded bypass capacitor filter arrangement (not illustrated) housed within a box-like conductive enclosure member 60. This member is secured to a magnetic pole piece plate member 62 of steel having an axially-aligned aperture 64 to provide for passage of the electrical leads. Leads 66 and 68 couple the RF shielded capacitor filter to the high voltage DC rectifying circuits. The enclosure member 60 is secured to the magnetic pole piece plate member by any suitable metallurgical techniques such as spot welding or soldering. Radiating cooling fins 70 contact the conductive envelope 52 and provide for cooling by circulation of air from a blower 23 to remove the heat generated by the high frequency oscillations.

The magnetic circuit includes permanent magnets 72 and 74 which may be of a circular or rectangular configuration, with the latter being illustrated. Any of the low cost barium ferrite ceramic-type magnets used in loud speakers can be utilized. The larger magnets 74 are disposed adjacent to the magnetic pole piece plate member 62 while the smaller magnets 72 are disposed adjacent to the tube magnetron envelope 52 in a stacked array. Four magnets have been shown in the illustrative embodiment which will provide the magnetic field flux density necessary for operation of magnetrons capable of generating 700 or 1,000 watts of continuous wave energy for microwave heating. An

axial passageway 76 is provided within the magnet members to accommodate the cathode support assembly 58. A substantially U-shaped housing member 78 of a magnetic material forms the path to orient the magnetic field flux lines in the interaction region within the magnetron envelope. To assist in the shaping of the magnetic field bucking magnets 80 are positioned as shown and are supported by the return path member 78.

Referring next to FIG. 4, the illustrative embodiment of the invention will be described. The combined RF energy escape seal, magnetic path and airflow pathdefining means comprises equidistantly spaced circular disc spacer members 82 contacting the magnetic pole piece plate member 62 on one surface and the larger permanent magnet 74 on the opposing surface. The size, number and thickness of the disc members 82 is selected to allow the flux lines 84 of the magnetic field (H) to be axially disposed. The circulating coolant may be introduced in the spaces 86 on all four sides as well as the passageway 76 and aperture 64 to cool the magnet members during operation of the magnetron. The flow of the circulating air is indicated by the arrows 88. The magnets 72 and 74 may be glued together to form the stack arrangement and steel disc members 82 may also be glued either to the magnetic plate member 62 or the magnet stack. The spacer members 82 also provide means for preventing the escape of any RF energy over the external electrical leads from the magnetron. The spaces 86, therefore, are selected to achieve this end by dimensions of one-quarter of a wavelength of the magnetron operating frequency.

An alternative structure for providing the aforementioned combined results is a unitary member of a low carbon steel magnetic material as illustrated in FIG. 5. In this embodiment planar plate member 90 has a substantial cruciform configuration defined by equidistantly spaced slots 92 extending radially and terminating in a closed end 94. All the slots 92 are interconnected at their inner ends in circular section 96 mating with the passageway 76 in the magnet members and the aperture 64 in the magnetic pole piece plate member 62. Each of the four quadrants resulting from this structure can be considered to be the equivalent of the circular disc spacer members 82 shown in the preceding views. The width as well as the length of the slots 92 is selected to prevent the escape of any RF energy in the operating frequencies of the magnetron generator as well as circulation of the coolant. The thickness will influence the magnetic field flux lines.

The foregoing structure provides a more efficient microwave magnetron of the permanent-magnet-type by a notable increase in magnetic circuit performance. The more efficient cooling of the magnet members results in the necessary magnetic flux density with fewer magnet members in the stack. The provision of the airflow path perpendicular to the flux lines also contributes to reducing the escape of any energy around the cathode end of the magnetron.

In view of the modifications, alterations or substitutions which will be apparent to those skilled in the art, it is intended that the foregoing detailed description of the preferred embodiments be considered in the broad- 65 est sense consonant with the appended claims.

I claim:

1. A microwave heating apparatus comprising: conductive wall members defining an enclosure;

a magnetron generator of electromagnetic energy having'an axial dimension and an anode, cathode and heater with external electrical leads for said cathode and heater;

means for coupling and distributing said energy within said enclosure;

means for circulating a cooling fluid medium along a path including said magnetron;

a magnetic field-producing circuit for operating said magnetron including external magnets and surrounding return path-defining members;

spacer means of a magnetic material defining a plurality of spaced fluid flow paths;

said spacer means being disposed between said magnets and one of said return path members with said paths being dimensioned to substantially prevent radiation of said energy adjacent to said leads.

2. The apparatus according to claim 1 wherein said magnetic field is principally oriented parallel to the magnetic axis and said fluid paths defined by said spacer means extend perpendicular to said magnetic field.

3. A microwave magnetron comprising:

an anode having an axial dimension;

a central cathode assembly within said anode having external electrical leads;

means for producing and directing a magnetic field principlally oriented parallel to the anode axis including an external magnet and magnetic return path members; and

spacer means of a magnetic material disposed between said magnet and one of said return path members to provide a plurality of spaced paths perpendicular to the anode axis for circulation of a cooling fluid and means to substantially prevent the radiation of microwave energy adjacent to said electrical leads.

4. The magnetron according to claim 3 wherein said spacer means comprise a substantially flat disc member defining a plurality of spaced radial slots and a central open section.

5. The magnetron according to claim 3 wherein said spacer means comprise equidistantly spaced flat disc members.

6. The magnetron according to claim 4 wherein said slots are closed at their outer ends. 

1. A microwave heating apparatus comprising: conductive wall members defining an enclosure; a magnetron generator of electromagnetic energy having an axial dimension and an anode, cathode and heater with external electrical leads for said cathode and heater; means for coupling and distributing said energy within said enclosure; means for circulating a cooling fluid medium along a path including said magnetron; a magnetic field-producing circuit for operating said magnetron including external magnets and surrounding return path-defining members; spacer means of a magnetic material defining a plurality of spaced fluid flow paths; said spacer means being disposed between said magnets and one of said return path members with said paths being dimensioned to substantially prevent radiation of said energy adjacent to said leads.
 2. The apparatus according to claim 1 wherein said magnetic field is principally oriented parallel to the magnetic axis and said fluid paths defined by said spacer means extend perpendicular to said magnetic field.
 3. A microwave magnetron comprising: an anode having an axial dimension; a central cathode assembly within said anode having external electrical leads; means for producing and directing a magnetic field principlally oriented parallel to the anode axis including an external magnet and magnetic return path members; and spacer means of a magnetic matErial disposed between said magnet and one of said return path members to provide a plurality of spaced paths perpendicular to the anode axis for circulation of a cooling fluid and means to substantially prevent the radiation of microwave energy adjacent to said electrical leads.
 4. The magnetron according to claim 3 wherein said spacer means comprise a substantially flat disc member defining a plurality of spaced radial slots and a central open section.
 5. The magnetron according to claim 3 wherein said spacer means comprise equidistantly spaced flat disc members.
 6. The magnetron according to claim 4 wherein said slots are closed at their outer ends. 